This invention relates to stretch wrap thermoplastic film, methods for forming a unitized plurality of goods, e.g., a pallet load, by application of stretch wrap film thereto and the stretch-wrapped units resulting therefrom. The invention is particularly concerned with a method of forming a thermoplastic stretch wrap film possessing a significant cling force on one surface thereof and no substantial cling force on the other surface thereof, the latter surface possessing a slide property when in contact with a like surface with relative motion therebetween due to the presence of particulate antiblock agent mechanically bonded to said surface.
The use of thermoplastic stretch wrap film for the overwrap packaging of goods and, in particular, the unitizing of pallet loads, constitutes a commercially significant application of polymer film. Overwrapping a plurality of articles to provide a unitized load can be achieved by a variety of techniques. In one procedure, the load to be wrapped is positioned upon a platform, or turntable, which is made to rotate and in so doing, to take up stretch wrap film supplied from a continuous roll. Braking tension is applied to the film roll so that the film is continuously subjected to a stretching, or tensioning, force as it wraps around the rotating load in overlapping layers. Generally, the stretch wrap film is supplied from a vertically arranged roll positioned adjacent to the rotating pallet load. Rotational speeds of from about 5 to about 50 revolutions per minute are common. At the completion of the overwrap operation, the turntable is completely stopped and the film is cut and attached to an underlying layer of film employing tack sealing, adhesive tape, spray adhesives, etc. Depending upon the width of the stretch wrap roll, the load being overlapped can be shrouded in the film while the vertically arranged film roll remains in a fixed position. Alternatively, the film roll, for example, in the case of relatively narrow film widths and relatively wide pallet loads, can be made to move in a vertical direction as the load is being overwrapped whereby a spiral wrapping effect is achieved on the packaged goods.
Some resins which have been used in the fabrication of stretch wrap film are polyethylene, polyvinyl-chloride and ethylene vinyl acetate. A fairly recent development has been the utilization of linear low density polyethylene (LLDPE) in the manufacture of stretch wrap film, e.g., as described in U.S. Pat. Nos. 4,399,180, 4,418,114 and 4,518,654, the contents of which are incorporated by reference herein. The excellent toughness and puncture resistance properties of LLDPE makes it an excellent resin for such an application. LLDPE and methods for its manufacture are described in, among others, U.S. Patent Nos. 3,645,992; 4,076,698; 4,011,382; 4,163,831; 4,205,021; 4,302,565; 4,302,566; 4,359,561; and, 4,522,987. Films fabricated from the typical LLDPE resins of commerce possess little or no cling property on either surface thereof in the absence of added cling agent.
Thermoplastic films possessing a cling property are known in the art. U.S. Pat. No. 4,311,808 describes a cling film containing a homogeneous mixture of polyisobutylene, ethylene-propylene and a low density polyethylene.
U.S. Pat. No. 4,367,256 describes a cling wrap plastic film based on a blend of high pressure low density polyethylene (HPLDPE) and LLDPE in which the latter resin represents from 5-16 weight percent of the total. In one embodiment, this film is sandwiched between two HPLDPE films.
U.S. Pat. No. 4,399,173 describes a multilayer plastic film free of melt fracture which is suitable for a variety of applications including, by implication, the stretch wrapping of goods. The film possesses a core layer of LLDPE resin of melt index 0.2-3.0 decigrams per minute and skin layers of LLDPE resin of melt index 5.0-20.0 decigrams per minute.
U.S. Pat. Nos. 4,399,180 and 4,418,114 describe a coextruded composite stretch wrap film in which an LLDPE core layer is surfaced with HPLDPE skin layers.
In the one-sided cling stretch wrap film of U.S. Pat. No. 4,518,654, layer A fabricated from a resin possessing an inherent cling property and/or a cling property resulting from the incorporation of a cling additive therein is coextruded with layer B fabricated from a resin exhibiting little if any cling, layer B exhibiting a slide property when in contrast with a layer of itself with relative motion therebetween. Layer B can contain from about 0.05 to about 2.0 weight percent of such antiblocking materials as crystalline and amorphous silicate, e.g., Na.sub.2 O.Al.sub.2 O.sub.3.SiO.sub.2.4H.sub.2 O, diatomaceous earth, talc, and the like, evenly distributed therein. In each of the two working examples of this patent, layer A is an LLDPE film containing a cling additive, namely, polyisobutylene, the aforesaid synthetic sodium silicate particles being uniformly incorporated throughout layer B of the film of Example 1 and amorphous silica particles being uniformly incorporated throughout layer B of the film of Example 2. As noted in U.S. Pat. No. 4,518,654, the presence of cling on one side of the stretch wrap film but non-cling/slide properties on the other overcomes the problem of the tendency of a pallet load overwrapped with a stretch wrap film possessing cling on both of its major surfaces from destructively pulling away from a similarly overwrapped pallet load with which it is in contact when one of the pallet loads is moved relative to the other (as would be the case in the fork lift truck-handling of such overwrapped pallet loads stored in a warehouse).
U.S. Pat. No. 4,436,788 describes a stretch wrap film obtained from a mixture of 40-90 weight percent ethylene-vinyl acetate copolymer and 8-55 weight percent LLDPE.
Japanese Laid-Open Patent No. 19528/1980 describes a two-layer stretch wrap film in which a low density polyethylene layer is coextruded with a low density polyethylene layer containing from 3 to 20 weight parts of a tackifier, e.g., a polyisobutylene having a molecular weight of 200 to 300.
As previously indicated, methods of stretch wrapping articles, containers, etc., are known. U.S. Pat. No. 3,986,611 describes a tension-wrapped palletized load obtained with a stretch wrap film possessing a cling additive.
U.S. Pat. No. 4,079,565 describes a stretch-wrapped package, process and apparatus which employs a stretch wrap polyethylene film.
U.S. Pat. No. 4,409,776 discloses a method and apparatus for packing articles with a composite stretch wrap film one surface of which is nonadhesive. The adhesive surface is obtained with an "adhesive film" such as one fabricated from ethylene-vinyl acetate copolymer, 1,2-polybutadiene or styrenebutadiene copolymer and the nonadhesive surface is obtained with a "nonadhesive film" such as one fabricated from a crystalline polyolefin, e.g., polyethylene, polypropylene or ethylene-propylene copolymer.
It is an object of the present invention to provide a thermpolastic stretch wrap film of simplified construction, e.g., a monolayer film, possessing a significant cling property on at least one of its major surfaces and a slide property on its other surface.
It is another object of the present invention to provide a stretch wrap film fabricated from an LLDPE resin inherently exhibiting a significant level of cling on one surface thereof in the absence of cling additive and a slide property on the opposite surface thereof due to the presence of particulate antiblock agent mechanically bonded to said surface.
Other objects of the invention include the use of a stretch wrap film of the aforementioned characteristics in the overwrapping of a plurality of goods, e.g., a pallet load, to provide a unitized packaged unit.